We shall be concerned with biochemical and cellular aspects of the secretory immune system, which is important in the local defense mechanisms of mucous membranes throughout the body. In the chemical work we shall be studying the structure of the secretory component portion of the secretory IgA antibody molecule and its mode of attachment to the remainder of the molecule. In the cellular work we shall be studying possible mechanisms underlying the dominance of IgA-type immunocytes (as compared to cells making other classes of immunoglobulins) in mucous membranes. We shall test the hypothesis that an interaction between cell surface IgA and locally produced secretory component is important in the preferential homing of IgA-immunocytes to mucous membranes.